The present invention relates to an electromagnetic actuator comprising a moving armature, at least one winding for electrical energisation and moving the armature, and an annular permanent magnet for providing magnetic latching, the permanent magnet being magnetised so as to have one pole face radially inwards of the other pole face. The annular magnet will normally extend over the path of movement of the centre of the armature, the armature being movable between two end positions. The latching effect is particularly useful with pulse-operated type actuators, and causes the armature to be positively retained in the end position it has reached until such time as the energisation of the winding(s) is reversed; this enables the actuator to be actuated by a short current pulse without requiring continuous energisation to retain the armature in its proper position.
FIG. 1 of British Patent Specification No. 1,089,596 discloses such an electromagnetic actuator.
A high coercivity magnet should be used for the annular magnet as the annular magnet must resist the coercive effect of the actuator windings, and this magnet can for instance be made of ceramic material. The magnet is difficult to fabricate and difficult to magnetise. A central pole-piece (usually in the form of a mandrel) must be used to obtain the radial magnetising field required, and the field strength of the magnetising field must be very high. As this pole-piece must be of reasonable size to obtain a strong enough magnetising field, the annular magnet could be made with an oversize bore, an annular iron packing piece being inserted into the bore to provide a central bore of suitably small size for the actuator. However, accurate machining is required to avoid an air gap between the annular magnet and the iron packing piece; in addition, in order to obtain sufficient volume of permanently magnetised material, the annular magnet must be relatively long or of large diameter.
The permanent magnet could be used for a purpose other than that of providing latching, for instance as described in relation to FIG. 1 of German Patent Specification No. 2,013,051.
It is desirable to provide an annular permanent magnet which is easy to fabricate and to magnetic without using an excessive amount of magnetised material.
According to the present invention, the annular permanent magnet is a composite magnet formed of at least two individual magnets which are magnetised so as to have one pole face radially inwards of the other pole face.
Although the individual magnets can be for instance bar magnets arranged radially around the axis of movement of the armature, the individual magnets are preferably substantially in the shape of sectors.
The magnetised sectors can be cheap to make and magnetise. Each individual sector can be formed for instance by a sintering process, and the effective radial thickness of the composite magnet can be easily determined by grinding or otherwise machining the inner arcuate surface of each sector in a jig which locates on the outside diameter (or outer arcuate surface) of the sector; the grinding can be performed using a formed peripheral grinding wheel with the sectors clamped in line. It is possible also to grind or otherwise machine the outside surfaces of the sectors if so desired, either as an alternative to machining the inside surfaces, or in addition to such machining; machining the inside, however, can result in the removal of less material for the same effect.
As no iron packing piece is required, the necessary magnetised volume can be obtained while keeping the composite magnet relatively short; for instance, its length (axial thickness) can be one-eighth to one-half of its internal diameter, preferably about 1/2 of its internal diameter. Its external diameter can be from 11/2 times to 4 times its internal diameter, and is preferably about double its internal diameter.
During magnetisation, the pole-piece adjacent the radially inner surface can be as large as is desired, within normal practical limits. The magnetisation of each individual permanent magnet can be parallel to the mean radius of the individual magnet, but it is possible to obtain more nearly radial magnetisation; near radial magnetisation is more desirable the smaller the number of individaul magnets, but in general terms, the field of the composite magnet does not have to be strictly uniform as long as it is generally radial.
There may be only two individual permanent magnets, which may be each approximately half annular, but there are preferably three or more individual permanent magnets, four individual magnets being found to be a useful number which can give the advantages of the invention without requiring too many parts to fabricate, magnetise and assemble; nonetheless, more than four permanent magnets may be used.
The individual magnets (preferably sectors) forming the composite magnet need not be in contact with each other and can be separated by radial webs which hold the magnets in position. In a convenient arrangement, the winding is or windings are wound on a bobbin which also mounts the magnets. The bobbin can thus have two portions on which two respective windings are wound, the portions being spaced apart axially and interconnected by radially-extending webs; the magnets are positioned between the bobbin portions and retained by the webs. An outer sleeve can surround the bobbin and hold the magnets in position. The bobbin can be arranged such that it can be removed from the actuator by releasing a simple, releasable fixing means, such as a coaxial nut, for easy removal of the bobbin, winding(s) and magnets.
Preferably, the winding(s) and the armature are coaxial with the path of movement of the armature and the individual permanent magnets are symmetrically arranged about the axis of the winding(s) and armature. This enables all the main parts (with the exception of the individual magnets and the separating webs) to be coaxial solids of rotation and enables the actuator to be cheap to manufacture.
The armature preferably moves in a space which is limited at each end by stationary ferromagnetic pole-pieces and whose sides are limited by a non-ferromagnetic tube interconnecting the pole-pieces, the composite annular magnet and the winding(s) being stationary and outside the tube, a ferromagnetic tube (acting as a magnetic yoke) surrounding the winding(s) and the composite annular magnet, and the ends of the ferromagnetic tube being closed by ferromagnetic end pieces. This is a useful constructional arrangement. In addition, by having the permanent magnets stationary, i.e. fixed, they are not subject to mechanical shock during the operation and there is thus considerably less tendency to demagnetise, or, if they are sintered, to break up.
The armature is preferably movable between two end positions, in each of which it abuts a third magnetic pole-piece and forms part of a substantially closed magnetic circuit. By having the magnetic circuit substantially closed, there is very little reluctance and a greater latching force. This reduces the susceptibility of the actuator to shocks of the kind causing it to change over when de-energised.
There are preferably two axially-spaced, coaxial windings with the composite annular magnet coaxially therebetween, the windings being dimensioned and electrically interconnected so that both are energised for change-over of the actuator and so that the induced fluxes in the composite annular magnet substantially cancel each other out, and this is most simply arranged by having the windings equi-spaced from the composite annular magnet and of the same size, and passing the same current through each winding. In this way, there can be little or no induced flux through the individual permanent magnets, and thus there is less tendency to de-energise the individual permanent magnets.
The actuator can in particular be used for actuating a valve such as a pilot valve, and the valve may be a valve as described and claimed in U.S.A. Pat. application Ser. No. 225569 filed 1st Feb. 1972.
The armature can if desired be a movable valve member, for instance carrying rubber seats, forming an electromagnetically-actuated valve.
The invention relates to a bistable, electromagnetically-actuated valve comprising three pressure fluid connections and two valve orifices connected to two respective pressure fluid connections, the armature being movable between two end positions such that a first said pressure fluid connection is connected to the third said pressure fluid connection when the armature is at one end position, and the second said pressure fluid connection is connected to the third pressure fluid connection when the armature is at the other end position. This bistable, electromagnetically-actuated valve provides a very simple bistable valve, particularly for pilot valve use, and although it is preferred that the annular permanent magnet is a composite annular permanent magnet, this need not necessarily be so.